Devices have long been known which allow the removable locking of mechanical members onto shafts and which provide a split bush having a conical external surface and a cylindrical internal surface. At least a pair of hollows suitably located on the external surface of the bush, in positions corresponding to tapped hollows on the bore of the mechanical member, allows the same bush to be fixed to the hub of the mechanical member by means of bolts or grub-screws. On the external surface of the bush it is further provided a tapped hollow, in a position corresponding to a smooth hollow on the bore of the mechanical member, in order to allow the removal of the bush from the mechanical member by means of one of the two previously removed grub-screws.
A keyway is provided on the internal surface of the bush and on the external surface of the shaft to allow the same bush to be fixed to the shaft. An example of such a known device is commercially available under the trade name SER-SIT.RTM..
The key is not very suitable in the cases in which the mechanical member must be locked in position distant from the ends of the shaft because of the difficulties of working keyways in such positions. Furthermore, the fixed position of the keyway constitutes a limitation of the angular positioning of the mechanical member on the shaft.
The presence of a keyway on the shaft, beyond requiring suitable working, weakens the structure of the some shaft. Therefore, for parity of transmitted torque, it is necessary to provide shafts with wider total section to give the necessary effective section.
Then it is also known that coupling by means of keys causes the formation of fretting corrosion due to the clearance (which increases with the time) between the shaft and the bush fixed on with the key. The clearances generated between bush and shaft causes a loss of precision in the angular positioning of the mechanical member with respect to the shaft and can also cause the break-up of the same key.
Furthermore, the need of providing a tapped hollow on the external surface of the bush requires a long and fine working and a considerable waste of material. Indeed, in order to obtain such a bush, it is necessary to drill and thread a piece having a greater diameter with respect to that of the final bush and then to turn the external surface of the piece until the tapped hollow on the conical external surface is obtained. It is evident that this involves considerable costs of production.
Friction-based locking bushes are also known, which are used for locking mechanical members having a cylindrical bore, comprising at least a pair of taper split bushes.
For allowing the transmission of the torque by friction, the locking bushes of this type develop notable radial pressures which bears on the hub of the mechanical member. Therefore, these locking bushes require mechanical members with very wide hubs compared to the section of the shaft (diameter up to four times the diameter of the shaft for hubs made from cast iron type G-20) and dependent in any case on the yield point of the material of the hub.
Locking bushes of this type can also have dismantling problems owing to the locking of the two split bushes onto the respective coupling surfaces. Until today, although these disadvantages have been known for some time, no suitable product to overcome the cited drawbacks has been proposed to the knowledge of the Applicant.
The above disclosed disadvantages of the known devices cause notable problems, both technical and of cost, and therefore there is a long time need for a technical solution which allows such drawbacks to be overcome.